Parents are becoming increasingly concerned that their children may be harmed when they are out of their sight. Almost daily the media reports events involving small children being harmed when the small children have wandered from sight of their parents. Furthermore, in today's increasingly mobile society families with small children regularly visit malls, amusement parks and other public places where crowds of people are found which provide an environment where small children can be harmed or become lost or wander from sight of their parents because of their natural inquisitiveness, tendency to explore their surroundings, or their desires to be free from control of their movements by their parents.
Devices are commercially available to limit or monitor movements of children. Devices exist for tethering children to their parents. Further radio systems are commercially available which generate an alarm when children move outside a radius from a radio receiver which receives transmissions from a transmitter worn by children. The tethering devices have a limited restraint radius and create animosity between a child and the parents. The radio systems have a fixed radius of approximately fifteen feet which is too small to permit useful monitoring if a parent does not wish to totally keep a child in sight and cannot be used for tracking.
Numerous radio tracking systems have been proposed which utilize radio communications to locate a mobile radio transmitter and/or to determine when a mobile radio transmitter carried by a person has exceeded a set range measured from a radio receiver. These systems have one or more radio transmitters which broadcast a coded identification of each radio transmitter which is received by a radio receiver and processed to determine the distance and, in some of these systems, the direction between each transmitter and receiver. See U.S. Pat. Nos. 4,785,291, 5,115,223, 5,119,072, 5,245,314, 5,289,163, 5,307,053 and 5,357,259, Patent Application WO 87/06748, U.K. Patent Application GB 2182183A and Japanese Patent Application No. 64-311842. A wide range of implementations of radio tracking systems are described in the above-referenced patents and published applications.
The determination if a mobile radio transmitter has moved out of range from a radio receiver receiving an identification code of the radio transmitter is accomplished in many different ways in these patents and applications. Two ways which are described for determining if a mobile transmitter has moved out of range are by determining if the received identification code signal has dropped below a predetermined signal strength or the received identification code signal has not been received for an elapsed time interval.
Radio communication systems which are designed to determine when a mobile transmitter worn by a person has moved outside of a set range and/or to track a person encounter severe problems because of (1) limitations of transmitter power imposed by the Federal Communications Commission which limit broadcast power below 100 milliwatts, and (2) various environmental factors which cause interference, fading, or signal attenuation of the identification code signal which is periodically sent from the mobile radio transmitter to the monitoring radio receiver. The transmitter identification code signal may be severely attenuated by passage through the bodies or body parts of people or other structures in the line of site between the radio transmitter and the radio receiver. The presence of people and structures in the line of sight causes substantial attenuation of the transmitted identification code signal which may cause the identification code of the radio transmitter to be periodically or permanently attenuated below the discrimination level of the radio receiver causing a false indication that the mobile radio transmitter has moved out of a set range and an inability to further track the mobile radio transmitter.
Furthermore, natural fading phenomena, such as Rayleigh fading, which is a function of the transmitting frequency and the relative velocity between the mobile radio transmitter and radio receiver are severely aggravated by low speed movement, such as when a child or patient is walking with a transmitter attached to their person to facilitate their tracking. These fading phenomena affect the determination if a set range has been exceeded and a direction determination of the transmitter relative to the receiver. Additionally, other man-made interferences, such as electrical noise and multipath interference caused by buildings, can periodically cause the identification code signal transmitted from the radio transmitter to be attenuated to a level below the discrimination level of the radio receiver tracking the transmitter which also causes a false indication that the radio transmitter is outside a set range and/or the inability to track the direction of the radio transmitter movement relative to the radio receiver with a directional antenna.
Error correction code may be transmitted in a frame of bits encoding the identification code of the radio transmitter. One or more frames encoding the identification code of the transmitter may each contain a set number of error correction code bits which are processed by the radio receiver to correct minor bit errors such as one or two bits which occur within the identification code frame bits. One well known error correction code for accomplishing this function is the BCH code.
The serial processing of the bits of frames which contain error correction code is typically implemented with a series of EXCLUSIVE OR gates. When a number of bit errors in a frame exceeds the error correction capacity of error correction code, the data within the frame is erroneous. The prior art methods of wireless data transmission do not permit the recovery of valid data bits from a frame containing a number of bit errors which exceed the bit error correction capacity of the error code therein which error correction capacity, for most types of error correction codes, is two bits.
The cumulative effects of mis-synchronization of a radio receiver to receive transmissions from radio transmitters, Rayleigh fading, and man-made noise noticeably reduces the reliability of current digital radio receivers to receive error free data. A gap in a data transmission in excess of 1 millisecond may cause a radio receiver to terminate the receiving process. In a situation of tracking a radio transmitter with a radio receiver which receives a periodic digital transmission of the radio transmitter's identification code, termination of the receiving process results in the correct identification of the radio transmitter not being received. As a result, the transmission from a radio transmitter which is, in fact, within a set range of a radio receiver which is monitoring the distance of the radio transmitter from the radio receiver is falsely received as being out of range. This results in an erroneous condition of monitoring the distance of the radio transmitter from the radio receiver and further, may cause a panic situation or otherwise cause the person using the radio receiver to not trust the reliability of the radio tracking system.
An analysis of wireless prior art data transmission protocols in accordance with accepted mathematical relationships for their evaluation reveals that they are poorly suited for data transmissions of more than a few characters in length. The following mathematical relationships are used to analyze fading: